Several approaches towards miniaturization for liquid phase analysis have developed in the art; the conventional approach using drawn fused-silica capillary columns.
In conventional miniaturized technology the instrumentation has not been reduced in size; rather, it is the separation compartment size that has been significantly reduced. As an example, micro-column liquid chromatography (μLC) has been described wherein columns with diameters of 100-200 μm are employed. Another approach towards miniaturization has been the use of capillary electrophoresis (CE) which entails a separation technique carried out in capillaries 25-100 μm in diameter. Both of the above-described “conventional” miniaturization technologies (μLC and CE) represent a first significant step toward reducing the size of the chemical portion of a liquid phase analytical system.
One major drawback in the current approach to miniaturization involves the chemical activity and chemical instability of silicon dioxide (SiO2) substrates, such as silica, quartz or glass, which are commonly used in both CE and μLC systems. More particularly, silicon dioxide substrates are characterized as high-energy surfaces and strongly adsorb many compounds, most notably bases. The use of silicon dioxide materials in separation systems is further restricted due to the chemical instability of those substrates, as the dissolution of SiO2 materials increases in basic conditions (at pHs greater than 7.0).
To avoid the problems arising from the inherent chemical activity of silicon dioxide materials, prior separation systems have attempted chemical modifications to the inner silica surface of capillary walls. In general, such post-formation modifications are difficult, as they require the provision of an interracial layer to bond a desired surface treatment to the capillary surface, using, for example, silylating agents to create Si—O—Si—C bonds. Although such modifications may decrease the irreversible adsorption of solute molecules by the capillary surfaces, these systems still suffer from the chemical instability of Si—O—Si bonds at pHs above 7.0. Accordingly, chemical instability in SiO2 materials remains a major problem.
However, despite the recognized shortcomings with the chemistry of SiO2 substrates, those materials are still used in separation systems due to their desirable optical properties. In this regard, potential substitute materials which exhibit superior chemical properties compared to silicon dioxide materials are generally limited in that they are also highly adsorbing in the UV region, where detection is important.
Although silicon micromachining and etching have been useful in the fabrication of miniaturized analysis systems, there are significant disadvantages to the use of this approach in creating the system. Initially, silicon micromachining is not amenable to producing a high degree of alignment between two etched or machined pieces. This has a negative impact on the symmetry and shape of a separation channel formed by micromachining, which in turn may impact separation efficiency. Secondly, sealing of micromachined or etched silicon surfaces is generally carried out using adhesives which may be prone to attack by separation conditions imposed by liquid phase analyses. Furthermore, under oxidizing conditions, a silica surface is formed on the machined or etched silicon substrate. In this regard, silicon micromachining and etching are limited by the chemistry of SiO2. Accordingly, laser ablation techniques have been described in commonly owned U.S. Pat. Nos. 5,571,410 and 5,658,413 to Kaltenbach et al., the disclosures of which are incorporated by reference in their entirety, for preparing miniaturized analysis devices that address these problems.
Currently, masks for laser ablation primarily are used to define the laser illumination such that features of constant depth or through holes are ablated in the substrate to be modified. However, there are some applications that may require the various features or holes in a single substrate to have depths that are different from one another. For example, both ablated channels and through holes may be desired in a single substrate. In this case, it is not possible to perform this ablation using a single conventional mask and multiple conventional masks have been required. There are several different techniques that can be used to fabricate multiple depth parts, including the use of multiple masks, but it would be advantageous to perform the ablation of the patterns with a single mask for reasons of cost, fabrication time, alignment, and simplicity. It may be of use, however, to examine previous mask technologies used to create single depth parts. The following is a brief discussion of the fabrication and use of conventional laser masks.